1. Field of the Invention
The present invention relates to an access method, a mobile station and a base station for a CDMA (Code Division Multiple Access) mobile communication system, in which a plurality of mobile stations access a base station at an optimal timing using a common channel.
2. Background
In conventional mobile communication systems, a packet communication is used in the transmission of control signals and the like. For example, in the digital type automobile telephone service standard (RCRSTD-27C) in Japan, it is prescribed that when a mobile station calls, the mobile station transmits a calling signal (calling packet signal) by using an access channel which can be used by all mobile stations in common. However, in a TDMA (Time Division Multiple Access) mobile communication system, in the case where a plurality of mobile stations transmit the calling signal at the same time, calling packets may `collide` in the common channel, causing transmission efficiency to deteriorate.
The above mentioned example is a problem which occurs in the case of a TDMA mobile communication. In the case of a CDMA mobile communication system in which each channel is divided by codes, however, the base station can receive all calling packets even if a plurality of mobile stations transmit calling packets at the same time. When many channels are used at the same time, however, mutual interference between signals increases, so that the base station cannot receive all calling packets. This phenomenon will be referred to as a "collision" in the following description. Therefore, even in the case of CDMA mobile communication, when many mobile stations transmit calling packets at the same time, the transmission efficiency deteriorates due to the collision, in the same way as occurs in TDMA mobile communication systems (See CDMA digital mobile communication method (TIA IS-95) in the US for example).
As described above, packet collisions may occur even in the case of CDMA mobile communications, so that when the number of channels being used in the common channel reaches a prescribed number, new packet transmissions must waited until the number of the channels in use decreases, i.e., until the transmission of packets from other mobile stations is completed. However, it is not possible for the mobile stations which are waiting to transmit to know when the transmission of packets from other mobile stations is finished, so that the waiting mobile station must either retry transmitting the calling packets at fixed intervals, or must constantly observe the condition of the channel. In the case of retrying transmission at fixed intervals, however, transmission efficiency deteriorates due to the collision of packets. Furthermore, in the case where the mobile station observes the condition of the channel, it is necessary that the mobile station receives a transmitted signal from the other mobile stations. However, constant observation is not possible since other mobile stations may frequently go outside the vista. In addition, a considerable load is imposed on the mobile station, even if the mobile station can constantly observe the condition of the channel.